Well done OP, really well done! Today's just a day. We've never bothered with the commercialised fakery.
That new single status sends a great message - to everyone.
And you were right to tell his friend's wife - chances are he's up to something too, and they were each other's alibis.
Re the MIL - she'll be panicking over maybe losing access to her grandchildren and will support any attempt he makes to wheedle back in. It's understandable - but the unit that is you and your children comes first.
You're better off free of him.
And as Shakespeare says, your best message to the OW is 'I wish you all joy of the worm'. Her lovely exciting romantic adventure is turning very sour indeed, as she realises the truth. That poor blameless baby.
You've the usual sensible 'ducks in a row' MN advice from women who've been there, and a ready made community of hand holders who can support you through this very old and well trodden journey.
All the very best to you all and your brilliant supportive family and friends.